Detection device for a spectrophotometer

ABSTRACT

The present invention relates generally to a detection device for a spectrophotometer system, and in particular to a light detection device with an analogue input and a digital output. The object of the present invention is to provide a more accurate detection device, which requires substantially less space for accommodating the detection device in a spectrophotometer. This object is solved by using a successive approximation A/D converter ( 16 ) which has internal sample and hold circuits, and which heretofore has been used in the audio industry. This will increase the accuracy of the detection device and make it more resistant against disturbances. It will also decrease the space required for such detection device, down to one third compared to conventional detection devices.

BACKGROUND OF THE INVENTION

1.Field of the Invention

The present invention relates generally to a detection device for aspectrophotometer system, and particularly to a light detection devicewith an analogue input and a digital output.

2. Description of the Prior Art

In a spectrophotometer system the concentration of a substance in a testsample is measured. Such a system generally comprises a continuous lightsource, optical elements and a detection device for detecting a testsample beam of light and/or a reference beam of light. The advantage ofdetecting both a test and a reference beam of light, i.e. with a twochannel system, is that the measurement of the concentration of thesubstance is made independently of the variations of the light source.In a two channel system the test sample beam first enters a flow cell,through which test samples are passed from a chromatographic column, andthen impinges on a first detector with an intensity I and the referencebeam impinges directly, via optical elements, on a second detector withan intensity I₀.

The detector output signals are then processed in the detection deviceto firstly calculate the absorbency of light in the test sample inaccordance with the well known equation,

A=−log (I/I₀)

Thereafter the concentration in the test sample is calculated by usingthe following equation,

A=∈*1*C

where 1 is the path length of the flow cell, C is the concentration and∈ is the absorbability.

U.S. Pat. No. 4,678,917 describes a multichannel spectrophotometercomprising an array of photo detectors for obtaining data for a widespectrum chromatogram. The signals output by the photo detectors entersample and hold circuits before they, via a multiplexor switch, areconverted by an A/D-converter. The radiation source is a deuterium lampsending out continuous radiation through a sample cell. The object ofthis invention is to provide a simultaneous measurement for a widespectrum light, rather than for only one wavelength.

U.S. Pat. No. 4,318,618 describes an apparatus for automaticallymeasuring changing values of absorbency. The described system is a twochannel system having one measuring and one reference photo detector.The signals coming from the photo detectors are digitised via I/Vconverts and A/D converts before they are processed. The object of thisinvention is to secure that the absorption cell in which the sample iscontained is aligned in a measuring zone in order to provide an accuratemeasurement.

U.S. Pat. No. 5,387,979 shows another example of a two channelspectrophotometer system. This system uses a pulsating (modulated)source of radiation giving rise to an alternating current component.This reduces the measurement response time of the for changes in theconcentration of the specific substance measured.

U.S. Pat. No. 4,549,809 shows a one channel photometric measurementmethod, wherein readings are taken at precisely measured intervals as afunction of the width of the cuvette and at the same distance from theleading wall of each cuvette containing the test sample.

The detection devices for the spectrophotometers in the above describedU.S. Pat. Nos. generally comprise detectors, such as photo cells,integrator means, amplifying means, analogue to digital (A/D)converters, buffering means and evaluation means, such as amicroprocessor or computer.

The A/D converters that are used for such a detection devicetraditionally use external sample and hold circuits prior to theconverter. This causes problems, firstly in that the lines between thesample and hold circuits are subject to interferences and thus theaccuracy of the A/D conversion deteriorates. Secondly the sampling speedwill be affected negatively by using external sample and hold circuits.This again will deteriorate the accuracy of the A/D conversion due todark currents and charge injections.

The A/D converters in detection devices used for spectrophotometers arespecially designed and therefore manufactured in small series, and henceexpensive. The advantage of these specially designed A/D converters isthat they have many options and can be adapted to all kinds of signals.

Since there are two signals that have to be converted in the detectiondevice for the spectrophotometer, i.e. the reference signal and the testsample signal, two such A/D converters are required. Furthermore, theseA/D converters require several additional control circuits to operateproperly, due to all the available options mentioned above, andtherefore require a large space. Thus, there is a need for an accurateA/D converter that is less space consuming and much cheaper in order tobring down the overall size and cost for the detection device.

The above mentioned U.S. Patents furthermore all comprise a radiationsource sending out radiation continuously giving rise to dynamic outputwaveforms from the photo detectors, in which waveforms the peak valueshave to be determined in order to get an accurate measurement. Thecontinuous radiation does furthermore not prescribe any high demands onthe sampling frequency.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a moreaccurate detection device, which requires substantially less space foraccommodating the detection device in a spectrophotometer and also toreduce the manufacturing cost of such a device.

This object of the invention is solved by the device using a successiveapproximation A/D converter with an internal sample and hold circuit,which heretofore has been used in the audio industry, for example inDAT-tape recorders and other studio equipments. The use of such an A/Dconverter substantially reduces the space needed for the detectiondevice, down to one third of the space required for a conventionaldetection device. The configuration of such an A/D converter with aninternal sample and hold circuit enhances also the accuracy and thespeed of the converter. Furthermore such an A/D converter is muchcheaper, since it is mass produced.

The A/D converter according to the present invention is controlled by asequence generator in order to secure proper functioning of the A/Dconverter in the detection device for the spectrophotometer.

To further improve the accuracy of the detection device the radiationsource is a flash generating source giving rise to a step response fromthe photo detectors, of which step response only the start and endvalues need to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is described in greater detail with thehelp of a preferred exemplifying embodiment with reference to theaccompanying drawing in which,

FIG. 1 shows an embodiment of the detection device according to thepresent invention,

FIG. 2 shows the step response generated by the sensing means, thesignal of which is to be measured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a detection device for a spectrophotometer according to thepresent invention. The detection device comprises two sensing means Rand S, for example photo cells or other light sensing devices,integrator means 10, amplifying means 12, filtering means 14, asuccessive approximation A/D converter 16, buffering means 18, sequencegenerating means 20 and evaluating means 22, such as a microprocessor orcomputer.

It should be noted that even if the radiation source 15, which sends outthe radiation to be sensed by the photo detectors, is not in itself apart of the detection device, its way of operating will influence thedesign of the detection device. Compared to the above cited prior artthe detection device of the present invention is designed for usetogether with a flash generating source and not together with atraditionally continuous radiation source. The use of a flash generatingsource will give rise to a step response from the photo detectorsinstead of a traditional continuous waveform.

The two photo cells R and S are used to measure the light beams of thegenerated flash that impinge thereon, said light beams coming from thesame light source (not shown). The reference photo cell R detects theunaffected light coming from the light source, whereas the signal photocell detects the light that has passed a flow cell, through which thetest sample flows, whose concentration of a certain substance is to bemeasured. The outputs of the photo cells R and S will have an outputcurrent, i.e. the difference between the start and end current values ofthe step response, that is proportional to the amount of light impingingthereon.

The output current step responses from the photo cells are then fed toan integrator step 10 in which the currents are integrated. Since thereare two currents to be integrated this step comprises two separateintegrators. The integrators comprise an operational amplifier and acapacitor as is well known in the art.

The integrated currents are thereafter amplified in an amplifying step12. This step comprises two separate programmable amplifiers. Eachseparate amplifier consists of two cascade connected programmableamplifiers, such as AD 526.

Before the two detected signals enter the A/D converter 16 they arepassed through filtering means 14. Since the measuring range of the A/Dconverter 16 is between −2.7 and 2.7 volts and does not toleratevoltages outside the range of −5 to 5 volts, the signals leaving theamplifying step 12 have to be adapted in order to protect the A/Dconverter 16 and to keep the signal within the measuring range of theA/D converter 16. This is done by a second degree Butterworth filter andby applying an offset to the signal before it enters the A/D converter16. The filtering means 14 also comprise an overvoltage protection sothat the A/D converter 16 never can be fed with a signal that woulddamage it.

The A/D converter 16 is a successive approximation converter, such asPCM1750 from Burr-Brown, which heretofore has been used in the audioindustry. This A/D converter 16 is configured with internal sample andhold circuits, which enhance the accuracy and the speed of theconverter. Furthermore, the use of this A/D converter 16 willsubstantially reduce the space needed for the detection device, down toone third of the space required for a conventional detection device.This A/D converter 16 has two separate channels which are clocked andtrigged simultaneously by means of sequence generating means 20. Thecontrol signals for the A/D converter 16 are normally generated by adigital filter comprised in the A/D converter 16, but since the presentinvention does not use the linear filtration of the signal, the controlsignals have to be generated by said sequence generating means 20.

The sequence generating means 20 controls the whole measuring andtrigger procedure. It comprises two sequence generators, one slowsequence generator comprising a counter and a PROM and a fast sequencegenerator. The slow sequence generator controls among other things thestart of the A/D conversion and also control functions for the fastsequence generator. The fast sequence generator controls the A/Dconverter and the data flow coming out from it. It is believed that thedesign of the sequence generator 20 is within the skills of the man inthe art and design details are therefore left out for the sake ofsimplicity.

After the conversion the data are fed to a first in first out (FIFO)memory 18 via address controlled digital ports (not shown), such as two8 bit latches, that clock the data. The FIFO 18 comprises a series toparallel converter which groups the data from both channels and thenalternately writes the data into the FIFO memory 18. Also the FIFOmemory 18 is controlled by the sequence generating means.

At the end of the measuring sequence the data is the transferred fromthe FIFO 18 to the microprocessor 22.

FIG. 2 shows the step response generated by the sensing means R and S,the signal of which is to be measured and of which the essential parts30, 32, 34 are to be converted by the A/D converter 16. Since the objectof a spectrophotometer is to determine the concentration of a certainsubstance in a test sample only the start and the end values of the stepresponse are of interest. The sequence generating means 20 has a numberof prestored measuring sequences which can be selected by a user.

The measuring sequence starts with the generating of a flash from thelight source. This will cause the step response according to FIG. 2. Thesequence generating means 20 will control the A/D converter to takesamples at 30. Depending on which sequence the user has selected thenumber of samples will vary. Preferably eight, sixteen or even moresamples are taken and converted by the A/D converter. The reason why atleast 8 samples are taken is to eliminate the effect of noise and otherdisturbances.

The user could of course also select a smaller number of samples, butthis would affect the accuracy negatively.

In a corresponding way the sequence generating means 20 controls thesampling at 32 and 34 at the end of the step response. These sampled andconverted data are then transferred to the microprocessor 22 via theFIFO memory 18 as mentioned above. The measuring sequence is terminatedwhen the FIFO memory 18 has received the data. This enables a newmeasuring sequence to be started while the microprocessor 22 processesthe data.

The microcomputer 22 averages the values for each measuring point 30, 32and 34 and then calculates the concentration of the substance inquestion in the test sample.

Even though the present invention has been described with oneexemplifying embodiment, it is believed that the man skilled in the artcan make changes and modifications of the present invention withoutdeparting from scope of the invention, which is only limited by theattached claims.

What is claimed is:
 1. A detection device for a spectrophotometer, saiddevice having an analogue input and a digital output and comprisessensing means (R, S), integrator means (10), amplifier means (12), andA/D converter (16), and buffering means (18), all connected in series,and evaluation means (22), characterised in that said A/D converter (16)has internal sample and hold circuits and further comprising sequencegenerating means (20) for controlling sampling and conversion of a stepresponse generated by said sensing means (R, S).
 2. A detection deviceaccording to claim 1, comprising two separate measuring channels, one ofwhich measures a reference signal and the other measures a signalaffected by a test sample and wherein said evaluation means (22)comprises means for determining the absorbency of light in said testsample.
 3. A detection device according to claim 2, wherein saidsequence generating means generates clock signals to trigger said A/Dconverter to take samples of said reference signal and sample signal. 4.A detection device according to claim 1, wherein said sequencegenerating means is pre-set with at least one measuring sequencecomprising at least two measuring points (30, 32) of said step response.5. A detection device according to claim 4, wherein said measuringpoints (30, 32, 34) each comprises several samples of said stepresponse.
 6. A detection device according to claim 1, wherein the A/Dconverter (16) is a successive approximation converter.
 7. A detectiondevice according to claim 1, further comprising filtering means (14)connected between said amplifying means (12) and said A/D converter (16)for protecting said A/D converter (16) from overvoltages.
 8. A detectiondevice according to claim 1, wherein said buffering means comprises aseries to parallel conversion and a first-in/first-out memory (18).
 9. Adetection device according to claim 2, wherein said evaluating means(22) is a microprocessor for calculating the concentration of a detectedsubstance in said test sample.
 10. A spectrophotometer system,characterised by a detection device according to claim
 1. 11. Adetection device according to claim 6, further comprising a radiationsource.
 12. A detection device according to claim 11, wherein saidradiation source further comprises a flash generating radiation sourcefor providing a step response from said sensing means.
 13. A detectiondevice according to claim 12, wherein said sensing means furthercomprises a first and second photo detector.
 14. A detection deviceaccording to claim 13, wherein said first photo detector furthercomprises a reference photo cell and said second photo detector furthercomprises a signal photo cell.
 15. A detection device according to claim14, wherein said integrator means further comprises a first and secondintegrator.
 16. A detection device according to claim 15, wherein saidamplifier means further comprises a first and second programmableamplifier.
 17. A detection device according to claim 16, furthercomprising a filtering means serially connected between said amplifiermeans and said A/D converter, wherein said filtering means includesovervoltage protection circuitry to protect said A/D converter.
 18. Adetection device according to claim 17, wherein said A/D converterfurther comprises a first and second channel, said first and secondchannels being clocked and triggered by said sequence generating means.19. A detection device according to claim 18, wherein said sequencegenerating means further comprises a slow sequence generator and a fastsequence generator, wherein said slow sequence generator furthercomprises a counter and a PROM.
 20. A detection device according toclaim 19, wherein said buffering means comprises a series to parallelconversion and a first-in/first-out memory and wherein said evaluatingmeans is a microprocessor for calculating the concentration of adetected substance in a test sample.